Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.

When
this article on component (resistor, capacitor, and inductor) measurement
was written, readily available, inexpensive multimeters were not
in existence. For about $20 you can now buy a brand new handheld
DMM that will make very accurate resistance measurements and reasonably
good capacitance measurements at frequencies up to a few MHz, where
lead inductance starts to be significant (test frequency is usually
only a few kHz). Finding an affordable, accurate inductance meter
is another story. Cheap
LCR meters can be purchased on eBay, but don't be surprised
if the quality is not very good. The most accurate measurement method
uses a frequency in the realm of actual operation, and this article
presents methods that will allow you do do just that by using typical
bench top instruments.

Component Checking

By N. H. Crowhurst

A
discussion of various ways that circuit components in radio and
audio equipment can be checked without trouble.

Sometimes
the simpler things one encounters in radio and audio work are apt
to get overlooked. For example, it would seem to be quite an easy
matter to check the inductance of a smoothing choke or the capacitance
of an electrolytic capacitor, with the correct polarizing current
or voltage. However, when one looks around to find a test instrument
to make the measurement, it just isn't readily available, so we
are virtually forced into the routine of taking things for granted.

If we wish to check as to whether a certain component is
functioning correctly or not, the only available method seems to
be by substitution, using another component of the same type. Often
this proves to be somewhat unsatisfactory, because the results can
be inconclusive. We really need to know how to check the various
fundamental components used in radio: resistance, inductance, and
capacitance, to varying degrees of accuracy, ac­cording to their
purpose.

Resistance Values

The
simplest method of resistance checking is by means of a simple ohmmeter,
either an instrument built specifically for this purpose or an ohmmeter
range on a volt-ohm-milliammeter. Accuracy of this method of measuring
resistance rarely exceeds 10% and may not even be as good as this.

Assuming that the accuracy of the moving coil meter used
for the instrument is ±2% and that the resistors used in the instrument
are accurate to ±1 %, the accuracy of the instrument as a perfect
comparator between the internal and external resistances cannot
be better than ±1%. And the accuracy of comparison is only to within
±2% of the full-scale current reading on the scale. If the scale
reading, on a voltage or current scale, is compared with the reading
on the ohms scale, it will be found that an error representing 2%
of full scale in voltage or current reading may amount to an 8%
error in resistance value. This is at the point of maximum accuracy
of comparison, between the external resistance being measured and
the internal resistance of the instrument.

Thus it is seen
that the best accuracy obtainable using an instrument with a ±2%
movement and ±1 % internal resistance gives a guaranteed accuracy
at center scale reading of 9%. At readings between one-third and
3 times the resistance value, which is the range one might expect
to use before switching to the next scale, the accuracy can reasonably
be expected to stay within 10%. With an instrument using lower accuracy
components than those used for illustration, the accuracy of the
final reading in ohms will be considerably poorer than 10%.

From this it will be evident that an ohmmeter can only be used
to make a rough check as to whether a resistance is within the preferred
value range for which it is color coded - if it is of a ±10% or
higher tolerance rating. To check that the resistance is within
±10% of its rated value, the result is a little doubtful and it
is certainly impossible to rely on an ohmmeter reading to check
to a tolerance of ±5% or closer.

Although the ohmmeter readings
cannot be trusted for checking to close tolerances, it is possible
to use an ohmmeter to check for reasonably good matching between
pairs of resistors, if this happens to be the requirement rather
than close precision in actual value.

As an example, in
many push-pull amplifiers the resistors responsible for controlling
the gain in the two halves of the push-pull arrangement must be
closely matched to ensure balance. Production values may be specified
to 5% or even closer tolerances, to avoid the necessity of having
to select matched pairs, but the essential feature is that the value
of the two corresponding resistors shall be within a close tolerance
of one another. It will not necessarily matter if both of them are,
say, 10% or 15% from their nominal rating, as long as they are within
5% of each other. This the ohmmeter is reasonably capable of checking,
because it is quite possible to read an ohmmeter scale to within
5%. Since the question as to whether the reading is within 5% of
its actual value is unimportant in this particular application,
the significance of the reading does not matter as much as whether
the two resistors which should be matched give readings within 5%
of one another.

For some applications, however, such as
calibrated attenuators or instruments for use in radio it is necessary
to check resistor values to closer limits such as 5%, 2%, or even
1%, as the case may require. In these circumstances it is important
that the value shall really be within the specified percentage of
its rated value. The only method of making a measurement that is
satisfactory for this purpose is to use a Wheatstone type bridge,
using calibrated elements whose accuracy is better than the required
component accuracy.

For most radio purposes the Leeds &
Northrup bridge used for telephone line work is quite accurate enough.
In using a bridge there are two things that control the accuracy
of the reading obtained: (1) the accuracy of the resistance elements
of the bridge itself, and careful attention to see, that contact
resistance does not contribute an appreciable fraction under any
circumstances; and (2) the sensitivity of the null detector.

This second cause of inaccurate results can be checked by unbalancing
the bridge by a known percentage to see that an adequate off-balance
reading is obtained. Suppose, for example, the value required is
120,000 ohms, ±5%. Having balanced the bridge and obtained a null
at, say, 120,000 ohms, the resistance in the calibrated arm should
be altered by 5%, which represents a change of 6000 ohms.

If
clicking in 6000 ohms additional in the calibrated arm shows appreciable
deflection, then the reading may be regarded as accurate; but if
the addition of 6000 ohms does not produce noticeable deflection
from balance on the null detector, the result is not reliable. To
improve its reliability one can either use a larger battery voltage
or source of supply to the bridge, or else get a more sensitive
null indicator.

Before leaving this discussion of resistance
values it should perhaps be emphasized that it is not wise to put
absolute trust in the color coding on a resistor. Occasionally even
the best resistor will be found incorrectly color coded. If the
error happens to be in the third color of the code, then the discrepancy
in resistance value will be a matter of shifting the decimal point
which can be quite serious. Also with some sets of coding colors
the difference between some of the colors is somewhat difficult
to determine, especially after the component has aged. For example,
orange and brown can get to look quite alike.

Usually the
first and second colors in the code can be identified by the combination
used, from the recognized preferred value range. If the first color
is blue, representing 6, the second color will most likely be either
red, representing 2, or gray, representing 8, because 62 and 68
are the preferred values in the 60 to 70 range. But there is no
such ready clue as to the likely color of the third band: it could
just as easily be brown or orange. Thus a resistor in which this
color looks at all doubtful could be either 620 ohms or 62,000 ohms,
which is a considerable difference!

This is where an ohmmeter
check can easily determine which of the two values is correct.

Inductance

Fig. 1. Bridge configurations for measuring inductance.
(A) the "Hay" bridge. (B) the "Maxwell" bridge. Relative
advantages of each type are discussed in the article.

Fig. 2. Modification of the "Hay" bridge to enable
it to measure inductance with polarizing current flowing.
Care is necessary not to exceed the dissipation rating of
the various bridge elements. See text.

Fig. 3. A simple inductance checker circuit for determining
inductance with the polarizing current flowing in the component.

Turning now to various kinds of inductance: the measurement of components
not intended for the passage of d.c. and without iron cores is a
fairly simple matter, with the aid of a conventional inductance
bridge. Using such a bridge, employing either the Hay or Maxwell
configuration (see Fig. 1), the inductance can be measured at a
frequency suitable for the purpose, with a method quite similar
to the operation of a bridge for measuring resistance.

The
principal difference is that two kinds of adjustment are usually
necessary to achieve null, because of the necessity for balancing
the bridge in both amplitude and phase. This enables the bridge
to give a reading of both inductance value and "Q" or loss factor.
Bridges of this type are clearly marked to indicate the correct
setting of the controls for making each kind of measurement.

There is usually no difficulty in achieving a null with the
air-core type of coil, but if the inductance employs any kind of
core, the null may not be quite as definitive, because of the distortion
of the injected test signal caused by the core. Also, if the generator
signal itself has any appreciable harmonic content, a Hay bridge
will never give a balance at both fundamental and harmonics at the
same setting. On the other hand, with an inductance where the only
loss is due to its resistance, such as occurs in an air-core coil,
the Maxwell bridge will give a fairly satisfactory balance for both
fundamental and the lower harmonic frequencies at the same setting.

When measuring an inductor that employs any kind of core
to increase the permeability, the magnetizing current is liable
to distort so the inductor itself will generate some harmonics not
present in the input from the generator. When the bridge is balanced
to the fundamental generator input, there will be a residual harmonic
present at the null point, generated by the inductor itself.

This is a good reason for using ear­phones if the generator
frequency is in the audio range. Otherwise an oscilloscope with
amplifier may be used as a null detector. It is then possible, listening
to the tone or looking at the trace, to determine when the fundamental
is balanced and the residue consists of harmonics.

But the
conventional type of bridge is only suitable for measuring induc­tances
where there is no polarizing current. The usual variety of smoothing
filter choke has to provide a specified inductance when polarizing
current is flowing and the inductance in the absence of such polarizing
current will be considerably higher than the rated inductance of
the choke with polarizing current. Unfortunately there is no simple
fixed relationship between these two values.

If the choke
has been designed to provide its maximum inductance at the polarizing
current for which it is designed, the air gap will be adjusted so
that, at this value of polarizing current, either reduction or increase
of the air gap would result in a reduction of inductance value.
However, in the absence of polarizing current, increasing the air
gap will always reduce inductance value, while reducing the air
gap will always increase inductance value.

From this simple
fact it is evident that measuring an inductance with no polarizing
current flowing is no criterion of its performance with polarizing
current. It can, of course, provide a check that the inductance
is not completely missing, due to short-circuited turns, in which
case the inductance might not even be adequate without polarizing
current flowing. But the fact that the inductance may measure twice
its required value with polarizing current is no evidence that the
choke will give its rated value with polarizing current.

Fortunately, with filter chokes of this nature close tolerances
are not too important. Usually a compliance with a minimum inductance
value will suffice.

It is sometimes possible to use a modified
Hay bridge, as shown at Fig. 2, to inject a polarizing current so
as to measure the inductance with the polarizing current flowing.
But this can be a dangerous procedure, because the polarizing current
may exceed the wattage rating of some of the internal components
of the bridge and cause permanent injury to it. It is, therefore,
better to devise a simple checking arrangement, as shown schematically
in Fig. 3.

This does not employ a bridge method, but checks
the inductance by injecting a known frequency and comparing the
a.c. voltage developed across the inductor with that across the
resistor in series with it. The relation between the a.c. components
of voltage developed will enable the approximate inductance value
to be calculated. This does not take into account the effect of
the inductor distorting the waveform of the a.c. signal component,
which invariably occurs in this type of inductor and is, in fact,
another reason why any attempt to produce a precise figure of inductance
will be somewhat meaningless. A rough check of this nature is quite
adequate for the purpose.

If 60 cycles is the supply frequency
for the a.c. component, dividing the calculated impedance of the
inductor by 377 will give the inductance value. For example, suppose
the series resistor used is 100 ohms (carefully checked in value),
and the a.c. voltages measured across the resistor and inductor
are 2 and 30 volts, respectively: then the impedance of the inductor
at 60 cycles is 1500 ohms, representing approximately 4 henrys.

Capacitance

Fig. 4. The "Drysdale" bridge which is used for measuring
capacitance. Refer to text.

Fig. 5. A simple bridge for capacitor checking that
forms the basis of a number of commercial units on the market.
The null detector is usually a "magic eye" tube.

Fig. 6. Modification of a "Drysdale" bridge to permit
the measurement of electrolytic capacitors with polarizing
voltage applied.

Fig. 7. Modification of the simple bridge of Fig.
5 to enable polarizing voltage to be applied to the electrolytic
capacitors.

For measuring all except electrolytic capacitors there are two methods,
which correspond in relative accuracy with the ohmmeter and bridge
methods used for measuring resistance.

The Drysdale bridge
(see Fig. 4) is a modified Wheatstone bridge, in which resistance
arms are used in the ratio positions, while a calibrated decade
capacitor is substituted for the calibrated resistance in the variable
standard arm. This type of instrument can give capacitance results
comparable to those obtained with the Wheatstone or Leeds &
Northrup bridge for resistance, but its use involves careful adjustment
of a number of controls until a null is achieved.

The alternative
method of capacitance measurement also uses a bridge, but one in
which the null is much more quickly achieved. In this bridge (see
Fig. 5) a standard capacitor is used in one arm, the unknown capacitor
in another arm, and a single potentiometer-type resistance for the
other two arms. This resistance is calibrated on the basis of the
ratio between the unknown and standard capacitors necessary to achieve
null.

With this type of bridge the unknown capacitor is
connected across the terminals of the bridge and the one dial turned
until null is indicated. The capacitance value is then read off
the dial. The accuracy of this type of instrument is usually comparable
to that of an ohmmeter, depending upon the accuracy with which the
potentiometer type resistance has been calibrated.

Neither
of these methods is really satisfactory for the measurement of electrolytics.
This can better be understood by discussing a little further the
behavior of electrolytic capacitors under different conditions.

In the first place, electrolytic capacitors freshly formed
ready for use, have a dielectric film on the active plate of the
correct thickness for the working voltage. Under this condition
the capacitor should have its rated capacitance.

But if
the capacitor is operated consistently at a lower polarizing voltage,
the thickness of the formed film will gradually deteriorate with
the result that the effective capacitance will increase somewhat.
This is not necessarily detrimental to the performance of the capacitor,
provided it is not subsequently required for service at its nominal
working voltage.

In much the same way electrolytic capacitors
kept in storage also show a deterioration in the dielectric film
resulting in an increase in effective capacitance. This means if
a six-month-old capacitor is taken from the shelf and measured on
a regular capacitance bridge, without applying the necessary polarizing,
it will probably show a value considerably in excess of its nominal
value. However, it will not be satisfactory for operation until
the electrolytic film has been formed up to the requisite thickness
for its working voltage.

This will have to be done with
the aid of a limiting resistor connected in series with the capacitor
to limit the polarizing current while the film is forming. Only
when the film has formed up so the voltage appearing across the
capacitor is at its working value without excessive leakage current
can its capacitance be measured to give a reliable indication of
its operating condition.

Also, if the capacitor is to be
installed in a piece of equipment for operation at its nominal working
voltage, it is vital that this reforming of the capacitor be performed
before installation, so the capacitor does not take an abnormally
high leakage current when the power is switched on and possibly
destroy itself before it has had a chance to become correctly reformed.

The correct measurement of electrolytic capacitors with
polarizing voltage applied can be undertaken with either type of
bridge, modified to a certain extent, as shown in the schematics
of Figs. 6 and 7. If the actual capacitance value of an electrolytic
capacitor is not vital, which often is the case, then all that is
necessary in installing a new one is to ensure that it is correctly
formed to its working voltage before connecting it in. This may
be done with the aid of the circuit shown in Fig. 8, which consists
of a high resistance feeding the capacitor with a voltmeter across
it to indicate when working voltage has been reached. The resistor
limits the leakage current through the capacitor to well within
the maximum leakage current allowed, and when the capacitor has
reached its nominal charged voltage, it can then be removed from
the charging arrangement. Then, after discharging the capacitor
for the sake of safety, the capacitor is ready for installation
in its intended circuit. Discharge should preferably be accomplished
through a fairly large resistor. The common practice of short-circuiting
a fully charged capacitor results in a very high discharge current
that may damage the capacitor.

Sometimes a capacitor which
has been in stock a long time will deteriorate in the quantity of
electrolyte present, so the capacitance will fall low in value,
even after it has been adequately reformed.

If it is not
convenient to build a capacitance measuring arrangement incorporating
the polarizing supply, a fairly legitimate result can usually be
achieved by ensuring that the capacitor is correctly formed using
the polarizing jig of Fig. 8, then discharging the capacitor and
finally measuring it immediately with the aid of one of the conventional
capacitance bridges without polarizing voltage.

If the electrolytic
capacitor is reasonably stable, a null will be obtained which will
not vary at a perceptible rate. If the capacitor is not sufficiently
stable to be reliable in use, the null may be observed to vary perceptibly
while the measurement is being taken. If the capacitance varies
at a rate that can be noticed while making the measurement, then
the capacitor should be discarded as insufficiently stable for reliable
operation.

The foregoing discussion has covered the more
common measurements necessary on resistance, inductance, and capacitance.
Sometimes much more precise methods of measurement are necessary,
especially where the equipment is for some kind of standard operation
such as a precision oscillator. In this kind of application it is
often necessary to make measurements, not only as to the precise
value at room or ambient temperature, but to determine the effect
of temperature on the component. To make such measurements, only
precision bridge apparatus is satisfactory, and the component should
be measured under carefully controlled conditions of temperature
and the measurements repeated at different temperatures, to discover
what temperature coefficient the component possesses. Fig. 8. Details
for constructing a simple jig for forming electrolytic capacitors
up to their working voltage. See article.

Fig. 8. Details for constructing a simple jig for forming
electrolytic capacitors up to their working voltage.

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas
and reference material while performing my work as an RF system and circuit design engineer.
The Internet was still largely an unknown entity at the time and not much was available
in the form of WYSIWYG
...

All trademarks, copyrights, patents, and other rights of ownership to images and text
used on the RF Cafe website are hereby acknowledged.